Historically there have been three general ways by which it was possible to produce satisfactory polyimide foams. According to the process described in Lavin et al U.S. Pat. No. 3,554,939, granted in January 1971, a monomer mixture composed of an ester of benzophenone tetracarboxylic acid and an aromatic polyamine in which the mixture has a volatile content of at least 9% is heated to a temperature at which foaming occurs contemporaneously with the polymerization of the ester and polyamine components until the polyimide foam is formed.
In another procedure exemplified by that described by Gagliani in Final Report NAS 9-14718 entitled "Fire Resistant Resilient Foams" dated February 1976, a mixture of diamines is added to an alcoholic solution of the half ester of benzophenone tetracarboxylic acid and reacted at 158.degree.-167.degree. F. (70.degree.-75.degree. C.) to form a heavy syrup which is heated in a circulating air oven at 180.degree. F. (82.2.degree. C.) for about 12-16 hours followed by drying in a vacuum oven at 176.degree.-194.degree. F. (80.degree.-90.degree. C.) for 60-90 minutes. Thereafter the polyimide precursor is pulverized into a powder which is spread over an aluminum foil on an aluminum plate and heated at 600.degree. F. (315.6.degree. C.) in an oven for 30 minutes to produce the foam. In a similar procedure reported by Gagliani et al in Final Report NAS 9-15050 entitled "Development of Fire-Resistant, Low Smoke Generating, Thermally Stable End Items for Aircraft and Spacecraft" dated June 1977, the dried precursor powder formed in about the same manner was subjected, inter alia, to a multi-stage technique in which the powder was placed in a pressure vessel positioned within an oven preheated at 232.2.degree. C. (450.degree. F.) and held at this temperature and at a reduced pressure (19.9-9.9 inches of Hg) for 15-30 minutes. The resulting foam was then postcured at 315.6.degree. C. (600.degree. F.) for 15-30 minutes in a circulating air oven.
The third procedure, which represented a distinct advance in the art, involves use of microwave radiation for converting the polyimide precursor into a cellular structure which normally is then subjected to final curing in a thermal oven. In actual practice the precursor is used in the form of a powder formed by spray drying an alcoholic solution of the monomers. See for example Gagliani et al U.S. Pat. Nos. 4,296,208; 4,305,796; 4,439,381; and 4,599,365; Final Report NAS 9-15050 (supra); Final Report NAS 9-15484 entitled "Development of Fire-Resistant, Low Smoke Generating, Thermally Stable End Items for Commercial Aircraft and Spacecraft Using a Basic Polyimide Resin", a report covering work conducted in the period December 1977 to April 1980; and Final Report NAS 9-16009 entitled "Formulation and Characterization of Polyimide Resilient Foams of Various Densities for Aircraft Seating Applications", a report covering work conducted in the period February 1980 to September 1981. In U.S. Pat. Nos. 4,305,796 and 4,439,381 it is indicated that the precursors may range in form from a `liquid resin` to a spreadable, pastelike formulation depending upon the nature and quantity of any fillers added to the resin.
While highly efficacious, spray drying and microwaving operations are time consuming and involve use of expensive process apparatus. A desirable contribution to the art would be a process in which high quality foams may be produced without need for use of microwave radiation and with reduced use of drying equipment such as spray dryers or vacuum dryers. This invention is believed to represent such a contribution.